The Mouse Mammary Tumor Virus (MMTV) long terminal repeat (LTR) contains the target site for steroid hormone action associated with this virus. Ultimately, in order to directly address the molecular mechanisms by which hormone-receptor complex, RNA polymerase II and other nuclear protein components interact with the MMTV LTR to produce the observed biological phenomenon of elevated rates of transcription, we feel that reconstitution of hormone regulated transcription in vitro will be required. In an attempt to develop a system capable of faithfully mimicing the in vivo situation, we have introduced MMTV LTR into eukaryotic cells as an episome, by using the Bovine Papilloma Virus (BPV) as a vector. The MMTV LTR, present at 200 copies per cell, is still responsive to steroid-specific induction of transcription in this episomal form. The chromatin structure of the MMTV LTR-containing minichromosomes has been examined by nuclease digestion and the minichromosomes can be purified from the bulk of chromosomal information based primarily on their small size. These minichromosomes should serve as useful templates for in vitro transcription studies.